A margin call occurs when the account equity falls below the used margin.
Once your margin level goes below 100%, your account will go into a margin call. You will no longer be able to open new positions. To avoid further losses or liquidation (which happens at 30% on our accounts), the client must either deposit more funds into the account or close some of the positions to raise back his margin level.
Our Invest account will not have margin calls. The coins are fully paid and so the margin is 100%.